In the manufacture of semiconductor wafers and integrated circuits, metal films are deposited on semiconductor substrates, oxides and other dielectrics. These metal films are used for different purposes, such as forming interconnect lines, metallic contacts, conductive vias and other conductive features on and above various surfaces of the semiconductor wafers.
Typically, aluminum has been a preferred conductive metal because it avoids various problems, such as a high contact resistance with silicon, which normally accompanies the use of gold and copper. Copper also suffers drawbacks compared to aluminum because copper migrates into device areas, causing problems in device performance. Aluminum, on the other hand, has good adhesion to silicon dioxide and performs well with plasma etching, as compared to copper.
Recently, greater interest has been shown by manufacturers of semiconductor devices in the use of copper and copper alloys for metallization patterns, such as in conductive vias and interconnects. Copper, compared to aluminum, has both good electromigration resistance and a relatively low electrical resistivity of about 1.7 .mu.ohm.multidot.cm. Unfortunately, copper is relatively quick to oxidize especially during processing or manufacturing of the integrated circuit. For example, a drawback with chemical mechanical polishing (CMP) is that copper layers and vias are prone to heavy oxidation, which can cause dishing during the chemical mechanical polishing step.
Additionally, dynamic recrystallization of electroplated copper is a potential difficulty because the microstructure continuously changes if the copper is not annealed. The changing copper microstructure can create additional problems during CMP. Conventional annealing of electroplated copper films results in the material being more ductile. Furthermore, the copper tends to lose mechanical strength and integrity during subsequent processes such as CMP, conventional passivation or inter-level dielectric (ILD) deposition. This often leads to stress-induced voiding and lack of adhesion of the copper film to underlying layers.